Electronic spill-out induced spectral broadening in quantum hydrodynamic nanoplasmonics

Orbital-Free Methods for Plasmonics: Linear Response

Plasmonic systems, such as metal nanoparticles, are widely used in different application areas, going from biology to photovoltaics.The modeling of the optical response of such systems is of fundamental importance to analyze their behavior and to design new systems with required properties.When the characteristic sizes/distances reach a few nanometers, non-local and spill-out effects become relevant and conventional classical electrodynamics models are no more appropriate. Methods based on the Time-Dependent Density-Functional Theory (TD-DFT) represent the current reference for the description of quantum effects. However, TD-DFT is based on knowledge of all occupied orbitals whose calculation is computationally prohibitive to model large plasmonic systems of interest for applications.On the other hand, methods based on the Orbital-Free (OF) formulation of TD-DFT, can scale linearly with the system size.In this Review, OF methods ranging from semiclassical models to the quantum hydrodynamic theory, will be derived from the linear response TD-DFT, so that the key approximations and properties of each method can be clearly highlighted. The accuracy of the various approximations will be then validated for the linear optical properties of jellium nanoparticles, the most relevant model system in plasmonics. OF methods can describe the collective excitations in plasmonic systems with great accuracy andwithout system-tuned parameters. The accuracy on these methods depends only on the accuracy on the (universal) kinetic energy functional of the ground-state electronic density. Current approximations and future development directions will be indicated.

Numerical scheme for a nonlinear optical response of a metallic nanostructure: quantum hydrodynamic theory solved by adopting an effective Schrödinger equation

Quantum hydrodynamic theory (QHT) can describe some of the characteristic features of quantum electron dynamics that appear in metallic nanostructures, such as spatial nonlocality, electron spill-out, and quantum tunneling. Furthermore, numerical simulations based on QHT are more efficient than fully quantum mechanical approaches, as exemplified by time-dependent density functional theory using a jellium model. However, QHT involves kinetic energy functionals, the practical implementation of which typically induces significant numerical instabilities, particularly in nonlinear optical phenomena. To mitigate this problem, we develop a numerical solution to QHT that is quite stable, even in a nonlinear regime. The key to our approach is to rewrite the dynamical equation of QHT using the effective Schrödinger equation. We apply the new method to the linear and nonlinear responses of a metallic nanoparticle and compare the results with fully quantum mechanical calculations. The results demonstrate the numerical stability of our method, as well as the reliability and limitations of QHT.

Landau Damping in Hybrid Plasmonics

The Landau damping (LD) mechanism of the localized surface plasmon (LSP) decay is studied for the hybrid nanoplasmonic (metal core/dielectric shell) structures. It is shown that LD in hybrid structures is strongly affected by the permittivity and the electron effective mass in the dielectric shell in accordance with previous observations by Kreibig, and the strength of LD can be enhanced by an order of magnitude for some combinations of permittivity and effective mass. The physical reason for this effect is identified as an electron spillover into the dielectric where the electric field is higher than that in the metal and the presence of quasi-discrete energy levels in the dielectric. The theory indicates that the transition absorption at the metal-dielectric interface is a dominant contribution to LD in such hybrid structures. Thus, by judicious selection of dielectric material and its thickness, one can engineer decay rates and hot carrier production for important applications, such as photodetection and photochemistry.

Superexchange Coupling-Induced Enhancements of Thermoelectric Performance in Saturated Molecules

To develop thermoelectric devices, it is of the utmost importance to design organic building blocks to have efficient thermopower. Whereas conjugated and aromatic molecules with intrinsic narrow band gaps are attractive candidates to achieve efficient thermoelectric properties, saturated molecules are usually avoided owing to intrinsically poor thermopower. Here we demonstrate that thermopower of saturated molecules can be enhanced by superexchange coupling. Specifically, thermoelectric properties of large-area junctions that contain self-assembled monolayers of oligo(ethylene glycol) thiolates and alkanethiolates are compared. Through large-area thermopower measurements using a liquid metal top electrode, we show that the superexchange coupling enhances the Seebeck coefficient and counterintuitively leads to an increase in the Seebeck coefficient with increasing the length in a certain conformation. The improved thermoelectric performance is attributed to the superexchange-induced enhanced ability to mediate metal wave function in junctions. Our work offers new insights for improving the thermoelectric performance of nonconjugated, saturated molecules.

Plasmon-Enhanced Resonant Photoemission Using Atomically Thick Dielectric Coatings

By proposing an atomically thick dielectric coating on a metal nanoemitter, we theoretically show that the optical field tunneling of ultrafast-laser-induced photoemission can occur at an ultralow incident field strength of 0.03 V/nm. This coating strongly confines plasmonic fields and provides secondary field enhancement beyond the geometrical plasmon field enhancement effect, which can substantially reduce the barrier and enable more efficient photoemission. We numerically demonstrate that a 1 nm thick layer of SiO₂ around a Au-nanopyramid will enhance the resonant photoemission current density by 2 orders of magnitude, where the transition from multiphoton absorption to optical field tunneling is accessed at an incident laser intensity at least 10 times lower than that of the bare nanoemitter. The effects of the coating properties such as refractive index, thickness, and geometrical settings are studied, and tunable photoemission is numerically demonstrated by using different ultrafast lasers. Our approach can also directly be extended to nonmetal emitters, to-for example-2D material coatings, and to plasmon-induced hot carrier generation.

Landau broadening of plasmonic resonances in the Mie theory

Landau damping in the metal nanosphere is considered beyond the quasistatic approximation with the use of the exact Mie theory when an incident plane wave can excite not only the dipole mode but also higher-order modes. In resonance approximation, when one considers excitation of a single mode, the analytical formula for the Landau damping coefficient for various modes has been derived. It was demonstrated that the simultaneous excitation of several eigenmodes, which are overlapped in the frequency domain, can lead to substantial correction of the Landau damping coefficients for the modes.

Optical properties of plasmonic core-shell nanomatryoshkas: a quantum hydrodynamic analysis

Plasmonic response of the metallic structure characterized by sub-nanometer dielectric gaps can be strongly affected by nonlocal or quantum effects. In this paper, we investigate these effects in spherical Na and Au nanomatryoshka structures with sub-nanometer core-shell separation. We use the state-of-the-art quantum hydrodynamic theory (QHT) to study both near-field and far-field optical properties of these systems: results are compared with the classical local response approximation (LRA), Thomas-Fermi hydrodynamic theory (TF-HT), and the reference time-dependent density functional theory (TD-DFT). We find that the results obtained using the QHT method are in a very good agreement with TD-DFT calculations, whereas other LRA and TF-HT significantly overestimate the field-enhancements. Thus, the QHT approach efficiently and accurately describes microscopic details of multiscale plasmonic systems whose sizes are computationally out-of-reach for a TD-DFT approach; here, we report results for Na and Au nanomatryoshka with a diameter of 60 nm.

An eigenvalue approach to quantum plasmonics based on a self-consistent hydrodynamics method

Plasmonics has attracted much attention not only because it has useful properties such as strong field enhancement, but also because it reveals the quantum nature of matter. To handle quantum plasmonics effects, ab initio packages or empirical Feibelman d-parameters have been used to explore the quantum correction of plasmonic resonances. However, most of these methods are formulated within the quasi-static framework. The self-consistent hydrodynamics model offers a reliable approach to study quantum plasmonics because it can incorporate the quantum effect of the electron gas into classical electrodynamics in a consistent manner. Instead of the standard scattering method, we formulate the self-consistent hydrodynamics method as an eigenvalue problem to study quantum plasmonics with electrons and photons treated on the same footing. We find that the eigenvalue approach must involve a global operator, which originates from the energy functional of the electron gas. This manifests the intrinsic nonlocality of the response of quantum plasmonic resonances. Our model gives the analytical forms of quantum corrections to plasmonic modes, incorporating quantum electron spill-out effects and electrodynamical retardation. We apply our method to study the quantum surface plasmon polariton for a single flat interface.